For a ciphering function it is generally required that the same input parameters must not be used twice. In other words, with same ciphering key, the combination of input parameters including a sequence number must be different for every encrypted/decrypted user plane packet.
In RRC (Radio Resource Control) protocol in UTRAN (UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network), a UE (User Equipment) provides a START value indicating the next number in a sequence to a target NodeB (NodeB is UTRAN base station) during handover. Thus, the sequence number is continuously used with every NodeB where the same keys are used. Therefore, the sequence number does not start from 0 in every handover. The mechanism to keep the sequence number synchronized is that the UE sends the START value which is the next number in the sequence. Hence, UE and target NodeB synchronize packet sequence numbers in handover.
In 3GPP LTE it has been decided that user plane packet sequence number is per bearer. If bearers are frequently created and destroyed, e.g. a new bearer is created/destroyed for every call, it is very likely that the same bearer will be used with the same ciphering key. The invention has been designed to overcome the above problems and to avoid frequent ciphering key re-negotiation for user plane stack.